OK, I tried wiring it like I said (with a separate neutral running to the device) and it didn’t make a bit of difference.
My theory is that the device’s small amount of power being derived off the L1 leg of the 220V feed makes the GFCI breaker think that a fault has occurred…
In my opinion, this is a major design flaw with this device given how much its being marketed towards pool/spa applications and given the legal/code requirements for GFCI circuits around such devices.
I’d really love a solution to this. Otherwise I’m going to have to return the device as it just won’t work for me.